generally the flour is a thickener - absorbing the juices and making a gravy.

cornstarch, potato starch, arrowroot - alternative thickeners

no reason oats and other cereals can't be used - as a rule they absorb moisture - which maybe the tricky part. if the mix does not have enough moisture to "cook" the grain/cereal it might not work out so well. for example raw rice - if it doesn't have enough moisture to cook it'll be quite crunchy.